Idiopathic low back pain and the therapeutic use of spinal manipulation make a significant impact clinically, socially, and financially. It has been estimated that more than 75 percent of us will experience low-back pain at some time in our lives costing an estimated $20-$50 billion annually. Recent findings from meta-analyses indicate that spinal manipulation can be recommended clinically for the treatment of acute and chronic idiopathic low back pain. Chiropractors deliver more than 90 percent of spinal manipulations performed in the United and annual expenditures on chiropractic services exceed $4.7 billion annually. Neither the causes of idiopathic low back pain nor the mechanisms underlying the physiological effects of spinal manipulation are well understood. Recent investigations by the applicant indicate that the mechanical history of a vertebra alters the sensitivity of lumbar paraspinal muscle spindles to subsequent vertebral movement. Muscle spindle discharge in response to vertebral gliding (translation) depended upon whether the vertebra had been previously held in a position that unloaded or loaded the spindle for as little as 5 seconds. We believe this novel finding in the spine is important because motion segments are continually subjected to passive forces that can fix their spatial orientation. The proposed experiments are based upon 3 hypotheses. (1) Sensitivity of paraspinal muscle spindles to vertebral translation increases when linear displacement of the vertebra has previously unloaded the paraspinal muscle spindle. (2) Sensitivity of paraspinal muscle spindles to vertebral translation decreases when linear displacement of the vertebra has previously loaded the paraspinal muscle spindle. (3) The therapeutic effects of spinal manipulation may be mediated, in part, by its ability to correct the positional-history-dependent changes in paraspinal muscle spindle sensitivity. History-dependent alterations in sensory feedback from paraspinal muscle spindles could lead to proprioceptive errors and adversely affect neural control of segmental motion. Knowledge of mechanical factors that affect sensory input from vertebral tissues will help provide a better understanding of the most appropriate preventive and therapeutic interventions for spinal care. Spinal manipulation is inherently a biomechanical intervention and the therapeutic successes of spinal manipulation for idiopathic low back pain may relate, in part, to the mechanical component of idiopathic low back.